Randomizing instanced objects
The "Object Properties -> Instancing" facility is pretty limited in terms of varying how the children are displayed.  If you want control over random variation, you probably want to create a particle system.  There are two particle system types, Emitter and Hair.  Generally speaking, emitter is used for animated particles (e.g., bubbles, smoke, etc.) while hair is used for static parts growing off of the structure (e.g., hair, leaves, stones, etc.).

It might be worth searching YouTube for "blender leaf particle system", which should give you lots of tutorials/examples.

Briefly, if you:


Create a thin, cylindrical "Branch" with applied scale, subdivided vertically to give it plenty of small faces.
Create a simple planar "Leaf" that's on a roughly one-unit (1m) scale
Add a new Hair particle system to the branch.
Set up the system:


Check "Advanced" to make "Rotation" panel available
Emission: Number=200
Rotation: Checked, Orentation Axis=Normal, Randomize=0.25
Render: Render As=Object, Scale Randomness=0.5, Instance Object="Leaf"



then that'll give you a start:



You can use vertex groups and weight painting to control the density on a mesh to make the distribution a little more realistic (or, in this case "low-poly-realistic"):



Here's a blend file: